U.S. patent number 8,254,175 [Application Number 12/638,836] was granted by the patent office on 2012-08-28 for semiconductor device and method of manufacturing the same.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Masaaki Higuchi, Tetsuya Kai, Hiroshi Matsuba, Yoshio Ozawa.
United States Patent |
8,254,175 |
Higuchi , et al. |
August 28, 2012 |
**Please see images for:
( Certificate of Correction ) ** |
Semiconductor device and method of manufacturing the same
Abstract
A semiconductor device includes a semiconductor region, a tunnel
insulating film formed on the semiconductor region, a
charge-storage insulating film formed on the tunnel insulating
film, a block insulating film formed on the charge-storage
insulating film, and a control gate electrode formed on the block
insulating film, wherein the tunnel insulating film comprises a
first region which is formed on a surface of the semiconductor
region and contains silicon and oxygen, a second region which
contains silicon and nitrogen, a third region which is formed on a
back surface of the charge-storage insulating film and contains
silicon and oxygen, and an insulating region which is formed at
least between the first region and the second region or between the
second region and the third region, and contains silicon and
nitrogen and oxygen and the second region is formed between the
first region and the third region.
Inventors: |
Higuchi; Masaaki (Yokohama,
JP), Matsuba; Hiroshi (Yokohama, JP),
Ozawa; Yoshio (Yokohama, JP), Kai; Tetsuya
(Yokohama, JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Tokyo, JP)
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Family
ID: |
42265823 |
Appl.
No.: |
12/638,836 |
Filed: |
December 15, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100157680 A1 |
Jun 24, 2010 |
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Foreign Application Priority Data
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Dec 16, 2008 [JP] |
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2008-320103 |
Dec 26, 2008 [JP] |
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2008-334636 |
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Current U.S.
Class: |
365/185.18;
257/324; 257/E29.309; 365/184; 438/591; 257/E21.021; 438/478 |
Current CPC
Class: |
H01L
29/40117 (20190801) |
Current International
Class: |
G11C
11/34 (20060101) |
Field of
Search: |
;365/185.18,184
;257/324,E29.309,E21.21 ;438/591,478 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2004-158810 |
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Jun 2004 |
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JP |
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2006-216215 |
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Aug 2006 |
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JP |
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2007-324545 |
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Dec 2007 |
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JP |
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Primary Examiner: Hidalgo; Fernando
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
Claims
What is claimed is:
1. A semiconductor device comprising a semiconductor region, a
tunnel insulating film formed on a surface of the semiconductor
region, a charge-storage insulating film formed on a surface of the
tunnel insulating film, a block insulating film formed on a surface
of the charge-storage insulating film, and a control gate electrode
formed on a surface of the block insulating film, wherein the
tunnel insulating film comprises: a first region which is formed on
the surface of the semiconductor region and contains oxygen and at
least one of silicon and germanium, as main components; a second
region which contains nitrogen and at least one of silicon and
germanium, as main components; a third region which is formed on a
back surface of the charge-storage insulating film and contains
oxygen and at least one of silicon and germanium, as main
components; and a fourth region which is an insulating region,
between the second region and the third region, a nitrogen
concentration in the fourth region being lower than a nitrogen
concentration in the second region, and an oxygen concentration in
the fourth region being lower than an oxygen concentration in the
third region, the second region is formed between the first region
and the third region.
2. The device of claim 1, wherein the nitrogen concentration in the
fourth region decreases from a boundary between the second region
and the fourth region toward a boundary between the third region
and the fourth region.
3. The device of claim 1, wherein the nitrogen concentration in the
second region and the fourth region decreases from a boundary
between the first region and the second region toward a boundary
between the third region and the fourth region.
4. The device of claim 1, wherein the nitrogen concentration in the
fourth region and the third region decreases from a boundary
between the second region and the fourth region toward a boundary
between the charge-storage insulating film and the third
region.
5. The device of claim 1, wherein the nitrogen concentration in the
second region, the fourth region and the third region decreases
from a boundary between the first region and the second region
toward a boundary between the charge-storage insulating film and
the third region.
6. The device of claim 1, wherein the tunnel insulating film
includes a fifth region, which is the insulating region, between
the first region and the second region, a nitrogen concentration in
the fifth region is lower than the nitrogen concentration in the
second region, and an oxygen concentration in the fifth region is
lower than the oxygen concentration in the third region.
7. A semiconductor device comprising: a memory cell transistor
including a semiconductor region, a tunnel insulating film formed
on a surface of the semiconductor region, a charge-storage
insulating film formed on a surface of the tunnel insulating film,
a block insulating film formed on a surface of the charge-storage
insulating film, and a control gate electrode formed on a surface
of the block insulating film; and a control circuit configured to
control the memory cell transistor, wherein the tunnel insulating
film comprises: a first region which is formed on the surface of
the semiconductor region and contains at least one of silicon and
germanium, and oxygen, as main components; a second region which
contains at least one of silicon and germanium, and nitrogen, as
main components; and a third region which is formed on a back
surface of the charge-storage insulating film and contains at least
one of silicon and germanium, and oxygen, as main components, and
the second region is formed between the first region and the third
region, and the control circuit is configured to apply a first
voltage, which is a positive bias, to the control gate electrode,
and thereafter to apply a second voltage, which is a negative bias
and has a smaller absolute value than the first voltage, to the
control gate electrode, in a write operation comprising a series of
operations of applying the first voltage and the second voltage,
and configured to apply a third voltage, which is a negative bias,
to the control gate electrode, and thereafter to apply a fourth
voltage, which is a positive bias and has a smaller absolute value
than the third voltage, to the control gate electrode, in an erase
operation comprising a series of operations of applying the third
voltage and the fourth voltage.
8. The device of claim 7, wherein the tunnel insulating film
includes an insulating region which is formed at least one of a
region between the second region and the first region and a region
between the second region and the third region, and contains at
least one of silicon and germanium, nitrogen and oxygen.
9. The device of claim 8, wherein the insulating region which
contains at least one of silicon and germanium, nitrogen and oxygen
is formed only between the second region and the first region.
10. The device of claim 8, wherein a nitrogen concentration in the
insulating region which contains at least one of silicon and
germanium, nitrogen and oxygen is lower than a nitrogen
concentration in the second region and higher than a nitrogen
concentration in each of the first region and the third region, and
a nitrogen concentration in the tunnel insulating film varies
stepwise toward the second region.
11. A method of manufacturing a semiconductor device comprising a
semiconductor region, a tunnel insulating film formed on a surface
of the semiconductor region, a charge-storage insulating film
formed on a surface of the tunnel insulating film, a block
insulating film formed on a surface of the charge-storage
insulating film, and a control gate electrode formed on a surface
of the block insulating film, wherein the tunnel insulating film
comprises a first region which is formed on the surface of the
semiconductor region and contains at least one of silicon and
germanium, and oxygen, as main components; a second region which
contains at least one of silicon and germanium, and nitrogen, as
main components; a third region which is formed on a back surface
of the charge-storage insulating film and contains at least one of
silicon and germanium, and oxygen, as main components; and an
insulating region which is formed at least one of a region between
the first region and the second region and a region between the
second region and the third region, and contains at least one of
silicon and germanium, nitrogen and oxygen, the insulating region
having a lower nitrogen concentration than the second region and a
lower oxygen concentration than the third region, and a formation
of the insulating region is performed at least one of a formation
between a formation of the first region and a formation of the
second region, and a formation between a formation of the second
region and a formation of the third region.
12. The method of claim 11, wherein the insulating region is formed
by oxidizing a surface of the third region or a surface of the
first region.
13. The method of claim 11, wherein the insulating region is formed
by nitriding a surface of the second region.
14. The method of claim 11, wherein the insulating region is formed
by an ALD method.
15. A semiconductor device comprising a semiconductor region, a
tunnel insulating film formed on a surface of the semiconductor
region, a charge-storage insulating film formed on a surface of the
tunnel insulating film, a block insulating film formed on a surface
of the charge-storage insulating film, and a control gate electrode
formed on a surface of the block insulating film, wherein the
tunnel insulating film comprises: a first region which is formed on
the surface of the semiconductor region and contains oxygen and at
least one of silicon and germanium, as main components; a second
region which contains nitrogen and at least one of silicon and
germanium, as main components; a third region which is formed on a
back surface of the charge-storage insulating film and contains
oxygen and at least one of silicon and germanium, as main
components; and a fourth region, which is an insulating region,
between the first region and the second region, a nitrogen
concentration in the fourth region being lower than a nitrogen
concentration in the second region, and an oxygen concentration in
the fourth region being lower than an oxygen concentration in the
third region; the second region being formed between the first
region and the third region.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from prior Japanese Patent Applications No. 2008-320103, filed Dec.
16, 2008; and No. 2008-334636, filed Dec. 26, 2008, the entire
contents of both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device and a
method of manufacturing the semiconductor device.
2. Description of the Related Art
At present, there has been developed a charge-trap-type nonvolatile
semiconductor memory device which uses, as a charge-storage layer,
a charge-storage insulating film for charge trap (see, e.g. Jpn.
Pat. Appln. KOKAI Publication No. 2004-158810). In this
charge-trap-type nonvolatile semiconductor memory device, the
charge, which has been injected in the charge-storage layer via a
tunnel insulating film, is trapped at a trap level in the
charge-storage insulating film, and thereby the charge is
accumulated in the charge-storage insulating film. As a typical
charge-trap-type nonvolatile semiconductor memory device, there is
known a MONOS type or SONOS type nonvolatile semiconductor memory
device, wherein a silicon nitride film, for instance, is used as
the material of the charge-storage insulating film.
In the above-described charge-trap-type nonvolatile semiconductor
memory device, there is proposed a tunnel insulating film having a
multilayer structure (ONO structure) comprising a silicon oxide
film, a silicon nitride film and a silicon oxide film, in order to
increase the charge erase speed (see, e.g. Jpn. Pat. Appln. KOKAI
Publication No. 2006-216215).
However, in the nonvolatile semiconductor memory device having the
above structure, a defect occurs at the interface between the
silicon nitride film and silicon oxide film due to a stress
occurring from the differences in inter-lattice distance and film
expansion coefficient between the silicon nitride film and silicon
oxide film. This defect becomes a trap site of electrons and holes.
The trapped electron or hole leaks into the semiconductor
substrate, causing deterioration in charge retention
characteristics of the charge-storage insulating film. Thus, it
cannot necessarily be said that there has been proposed a
nonvolatile semiconductor memory device having both excellent
charge erase characteristics and excellent charge retention
characteristics.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is
provided a semiconductor device comprising a semiconductor region,
a tunnel insulating film formed on a surface of the semiconductor
region, a charge-storage insulating film formed on a surface of the
tunnel insulating film, a block insulating film formed on a surface
of the charge-storage insulating film, and a control gate electrode
formed on a surface of the block insulating film, wherein the
tunnel insulating film comprises: a first region which is formed on
the surface of the semiconductor region and contains at least one
of silicon and germanium, and oxygen, as main components; a second
region which contains at least one of silicon and germanium, and
nitrogen, as main components; a third region which is formed on a
back surface of the charge-storage insulating film and contains at
least one of silicon and germanium, and oxygen, as main components;
and an insulating region which is formed at least one of a region
between the first region and the second region and a region between
the second region and the third region, and contains at least one
of silicon and germanium, nitrogen and oxygen, and the second
region is formed between the first region and the third region.
According to a second aspect of the present invention, there is
provided a semiconductor device comprising: a memory cell
transistor including a semiconductor region, a tunnel insulating
film formed on a surface of the semiconductor region, a
charge-storage insulating film formed on a surface of the tunnel
insulating film, a block insulating film formed on a surface of the
charge-storage insulating film, and a control gate electrode formed
on a surface of the block insulating film; and a control circuit
configured to control the memory cell transistor, wherein the
tunnel insulating film comprises: a first region which is formed on
the surface of the semiconductor region and contains at least one
of silicon and germanium, and oxygen, as main components; a second
region which contains at least one of silicon and germanium, and
nitrogen, as main components; and a third region which is formed on
a back surface of the charge-storage insulating film and contains
at least one of silicon and germanium, and oxygen, as main
components, and
the second region is formed between the first region and the third
region, and the control circuit is configured to apply a first
voltage, which is a positive bias, to the control gate electrode,
and thereafter to apply a second voltage, which is a negative bias
and has a smaller absolute value than the first voltage, to the
control gate electrode, in a write operation comprising a series of
operations of applying the first voltage and the second voltage,
and configured to apply a third voltage, which is a negative bias,
to the control gate electrode, and thereafter to apply a fourth
voltage, which is a positive bias and has a smaller absolute value
than the third voltage, to the control gate electrode, in an erase
operation comprising a series of operations of applying the third
voltage and the fourth voltage.
According to a third aspect of the present invention, there is
provided a method of manufacturing a semiconductor device
comprising a semiconductor region, a tunnel insulating film formed
on a surface of the semiconductor region, a charge-storage
insulating film formed on a surface of the tunnel insulating film,
a block insulating film formed on a surface of the charge-storage
insulating film, and a control gate electrode formed on a surface
of the block insulating film, wherein the tunnel insulating film
comprises a first region which is formed on the surface of the
semiconductor region and contains at least one of silicon and
germanium, and oxygen, as main components; a second region which
contains at least one of silicon and germanium, and nitrogen, as
main components; a third region which is formed on a back surface
of the charge-storage insulating film and contains at least one of
silicon and germanium, and oxygen, as main components; and an
insulating region which is formed at least one of a region between
the first region and the second region and a region between the
second region and the third region, and contains at least one of
silicon and germanium, nitrogen and oxygen, the insulating region
having a lower nitrogen concentration than the second region and a
lower oxygen concentration than the third region, and a formation
of the insulating region is performed at least one of a formation
between a formation of the first region and a formation of the
second region, and a formation between a formation of the second
region and a formation of the third region.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIG. 1A is a cross-sectional view taken in a channel length
direction (bit line direction) of a memory cell transistor of a
flash memory according to a first embodiment of the present
invention, and FIG. 1B is a cross-sectional view taken in a channel
width direction (word line direction) of the memory cell transistor
of the flash memory according to the first embodiment;
FIG. 2 shows a cross-sectional view of a tunnel insulating film in
the first embodiment of the invention, and a nitrogen concentration
distribution in the depth direction of a nitride film and an
oxynitride film;
FIG. 3A is an energy band diagram at a time of a charge erase
operation of a memory cell transistor of a comparative example of
the first embodiment of the invention, FIG. 3B is an energy band
diagram at a time of a charge erase operation of the memory cell
transistor of the first embodiment of the invention, and FIG. 3C is
an energy band diagram at a time after the charge erase operation
of the memory cell transistor of the first embodiment of the
invention;
FIG. 4A is a cross-sectional view taken along the bit line
direction of the semiconductor device according to the first
embodiment of the invention, and FIG. 4B is a cross-sectional view
taken along the word line direction of the semiconductor device
according to the first embodiment of the invention;
FIG. 5A is a cross-sectional view taken along the bit line
direction of the semiconductor device according to the first
embodiment of the invention, and FIG. 5B is a cross-sectional view
taken along the word line direction of the semiconductor device
according to the first embodiment of the invention;
FIG. 6A is a cross-sectional view taken along the bit line
direction of the semiconductor device according to the first
embodiment of the invention, and FIG. 6B is a cross-sectional view
taken along the word line direction of the semiconductor device
according to the first embodiment of the invention;
FIG. 7A is a cross-sectional view taken along the bit line
direction of the semiconductor device according to the first
embodiment of the invention, and FIG. 7B is a cross-sectional view
taken along the word line direction of the semiconductor device
according to the first embodiment of the invention;
FIG. 8 shows a cross-sectional view which schematically shows the
structure of a tunnel insulating film of Modification 1 of the
first embodiment of the invention, and a nitrogen concentration
distribution in the depth direction of an oxynitride film;
FIG. 9A is an energy band diagram at a time of a charge erase
operation of the memory cell transistor of Modification 1 of the
first embodiment of the invention, and FIG. 9B is an energy band
diagram at a time after the charge erase of the memory cell
transistor of Modification 1 of the first embodiment of the
invention;
FIG. 10 shows a cross-sectional view of a tunnel insulating film in
Modification 2 of the first embodiment of the invention, and a
nitrogen concentration distribution in the depth direction of a
nitride film and an oxynitride film;
FIG. 11A is an energy band diagram at a time of a charge erase
operation of the memory cell transistor of Modification 2 of the
first embodiment of the invention, and FIG. 11B is an energy band
diagram at a time after the charge erase of the memory cell
transistor of Modification 2 of the first embodiment of the
invention;
FIG. 12 shows a cross-sectional view of a tunnel insulating film in
Modification 3 of the first embodiment of the invention, and a
nitrogen concentration distribution in the depth direction of an
oxynitride film;
FIG. 13A is an energy band diagram at a time of a charge erase
operation of the memory cell transistor of Modification 3 of the
first embodiment of the invention, and FIG. 13B is an energy band
diagram at a time after the charge erase of the memory cell
transistor of Modification 3 of the first embodiment of the
invention;
FIG. 14A is a cross-sectional view taken in the bit line direction
of a memory cell transistor of a flash memory according to a second
embodiment of the present invention, and FIG. 14B is a
cross-sectional view taken in the word line direction of the memory
cell transistor of the flash memory according to the second
embodiment;
FIG. 15 shows a cross-sectional view of a tunnel insulating film in
the second embodiment of the invention, and a nitrogen
concentration distribution in the depth direction of an oxynitride
film, a nitride film and an oxynitride film;
FIG. 16 shows a cross-sectional view of a tunnel insulating film of
a comparative example of the second embodiment of the invention,
and a nitrogen concentration distribution in the depth direction of
a nitride film;
FIG. 17A is a cross-sectional view taken in the channel length
direction of a memory cell transistor according to a third
embodiment of the present invention, and FIG. 17B is a perspective
view of the memory cell transistor according to the third
embodiment of the invention;
FIG. 18A is a cross-sectional view, taken in the channel length
direction, showing the structure in which the memory cell
transistor according to the third embodiment of the invention is
provided continuous in the vertical direction, and FIG. 18B is a
cross-sectional view, taken in a direction perpendicular to the
channel length direction, showing the structure in which the memory
cell transistor according to the third embodiment of the invention
is provided continuous in the vertical direction;
FIG. 19A is a cross-sectional view taken along the channel length
direction of the semiconductor device according to the third
embodiment of the invention, and FIG. 19B is a cross-sectional view
taken along a direction perpendicular to the channel length
direction of the semiconductor device according to the third
embodiment of the invention;
FIG. 20A is a cross-sectional view taken along the channel length
direction of the semiconductor device according to the third
embodiment of the invention, and FIG. 20B is a cross-sectional view
taken along a direction perpendicular to the channel length
direction of the semiconductor device according to the third
embodiment of the invention;
FIG. 21A is a cross-sectional view taken along the channel length
direction of the semiconductor device according to the third
embodiment of the invention, and FIG. 21B is a cross-sectional view
taken along a direction perpendicular to the channel length
direction of the semiconductor device according to the third
embodiment of the invention;
FIG. 22A is a cross-sectional view, taken along the channel length
direction, showing the structure of a memory cell transistor
according to a fourth embodiment of the invention, and FIG. 22B is
a perspective view showing the structure of the memory cell
transistor according to the fourth embodiment of the invention;
FIG. 23A is a cross-sectional view, taken in the channel length
direction, showing the structure in which the memory cell
transistor according to the fourth embodiment of the invention is
provided continuous in the vertical direction, and FIG. 23B is a
cross-sectional view, taken in a direction perpendicular to the
channel length direction, showing the structure in which the memory
cell transistor according to the fourth embodiment of the invention
is provided continuous in the vertical direction;
FIG. 24A is a cross-sectional view taken along the channel length
direction of the semiconductor device according to the fourth
embodiment of the invention, and FIG. 24B is a cross-sectional view
taken along a direction perpendicular to the channel length
direction of the semiconductor device according to the fourth
embodiment of the invention;
FIG. 25A is a cross-sectional view taken along the channel length
direction of the semiconductor device according to the fourth
embodiment of the invention, and FIG. 25B is a cross-sectional view
taken along a direction perpendicular to the channel length
direction of the semiconductor device according to the fourth
embodiment of the invention;
FIG. 26A is a cross-sectional view taken along the channel length
direction of the semiconductor device according to the fourth
embodiment of the invention, and FIG. 26B is a cross-sectional view
taken along a direction perpendicular to the channel length
direction of the semiconductor device according to the fourth
embodiment of the invention;
FIG. 27 is a block diagram showing a memory cell transistor in a
flash memory according to a fifth embodiment of the invention;
FIG. 28A is a cross-sectional view, taken along the word line
direction, showing the memory cell transistor in the flash memory
according to the fifth embodiment of the invention, and FIG. 28B is
a cross-sectional view, taken along the bit line direction, showing
the memory cell transistor in the flash memory according to the
fifth embodiment of the invention;
FIG. 29 is a graph showing the relationship between a gate voltage
and time at a time of data write in the flash memory according to
the fifth embodiment of the present invention;
FIG. 30A is a cross-sectional view for explaining the operation of
the memory cell transistor in the flash memory according to the
fifth embodiment of the invention, and FIG. 30B is a
cross-sectional view for explaining the operation of the memory
cell transistor in the flash memory according to the fifth
embodiment of the invention, which follows the operation shown in
FIG. 30A;
FIG. 31 is a graph showing the relationship between a gate voltage
and time at a time of data erase in the flash memory according to
the fifth embodiment of the present invention; and
FIG. 32A is a cross-sectional view for explaining the operation of
the memory cell transistor in the flash memory according to the
fifth embodiment of the invention, and FIG. 32B is a
cross-sectional view for explaining the operation of the memory
cell transistor in the flash memory according to the fifth
embodiment of the invention, which follows the operation shown in
FIG. 32A.
DETAILED DESCRIPTION OF THE INVENTION
Semiconductor devices according to embodiments of the present
invention (charge-trap-type nonvolatile semiconductor memory
devices using, as charge-storage layers, charge-storage insulating
films for charge trap) will now be described. The semiconductor
devices according to the embodiments may be of a NAND type or a NOR
type, and these semiconductor devices are applied, in particular,
to a MONOS structure.
First Embodiment
Referring to FIG. 1A, FIG. 1B, FIG. 2, FIG. 3A, FIG. 3B and FIG.
3C, a description is given of the outline of the basic structure of
a semiconductor device according to a first embodiment of the
invention.
First, the structure of a memory cell transistor of a flash memory
according to the first embodiment is described. FIG. 1A is a
cross-sectional view taken in a channel length direction (bit line
direction) of the memory cell transistor of the flash memory
according to the first embodiment of the present invention, and
FIG. 1B is a cross-sectional view taken in a channel width
direction (word line direction) of the memory cell transistor of
the flash memory according to the first embodiment.
As shown in FIG. 1A and FIG. 1B, a tunnel insulating film 102 is
formed on the surface of a semiconductor substrate (silicon
substrate) 101. A charge-storage insulating film 103 is formed on a
surface of the tunnel insulating film 102. A block insulating film
104 is formed on the surface of the charge-storage insulating film
103, and a control gate electrode 105 is formed on the surface of
the block insulating film 104. A gate voltage for data write, erase
and read of the memory cell transistor MT is applied to the control
gate electrode 105. An interlayer insulating film 106 is formed so
as to cover the memory cell transistor MT. In addition, an element
isolation insulating film 107 is formed between memory cell
transistors MT which neighbor in the word line direction. A channel
region CH is formed on a surface of the semiconductor substrate
101, which is sandwiched between the element isolation insulating
films 107.
The tunnel insulating film 102 comprises an oxide film (first
region) 102a which is formed on the surface of the semiconductor
substrate 101; a nitride film (second region) 102b which is formed
on the surface of the oxide film 102a; an oxynitride film (fourth
region) 102c which is formed on the surface of the nitride film
102b; and an oxide film (third region) 102d which is formed on the
surface of the oxynitride film 102c.
Each of the oxide film 102a and oxide film 102d is, for instance, a
silicon oxide film containing silicon and oxygen as main
components. The nitride film 102b is, for instance, a silicon
nitride film containing silicon and nitrogen as main components.
The oxynitride film 102c is, for instance, a silicon oxynitride
film containing silicon, nitrogen and oxygen as main
components.
FIG. 2 shows a cross-sectional view of the tunnel insulating film
102 in the first embodiment, and a nitrogen concentration
distribution in the depth direction of the nitride film 102b and
the oxynitride film 102c.
As shown in FIG. 2, the nitrogen concentration in the oxynitride
film 102c decreases from the boundary between the nitrogen film
102b and the oxynitride film 102c toward the boundary between the
oxide film 102d and the oxynitride film 102c. Conversely, the
oxygen concentration increases from the boundary between the
nitrogen film 102b and the oxynitride film 102c toward the boundary
between the oxide film 102d and the oxynitride film 102c. A
nitrogen concentration distribution (A) is a distribution in the
case where the nitrogen concentration decreases at a constant
ratio. A distribution (B) is a distribution in the case where the
quantity of nitrogen is greater and the quantity of oxygen is
smaller than in the case of the distribution (A). A distribution
(C) is a distribution in the case where the quantity of nitrogen is
smaller and the quantity of oxygen is greater than in the case of
the distribution (A).
FIG. 3A is an energy band diagram at a time of a charge erase
operation of a memory cell transistor of a comparative example (a
tunnel insulating film has an ONO structure). FIG. 3B is an energy
band diagram at a time of a charge erase operation of the memory
cell transistor of the first embodiment. FIG. 3C is an energy band
diagram at a time after the charge erase (at a time of charge
retention) of the memory cell transistor of the first
embodiment.
As shown in FIG. 3A, in the case where the tunnel insulating film
102 has an ONO structure comprising an oxide film 102a, a nitride
film 102b and an oxide film 102i, the energy barrier against holes
is higher at the end of the valence band of the oxide film 102i
than the end of the valence band of the nitride film 102b. Thus, at
the time of the charge erase operation (at the time of hole
injection), the injection of holes in the charge-storage film 103
is blocked by the barrier of the oxide film 102i against the hole.
As a result, the holes are trapped in the potential well of the
nitride film 102b.
In the tunnel insulating film 102 of the present embodiment, the
oxynitride film 102c is formed between the nitride film 102b and
oxide film 102d. Thus, as shown in FIG. 3B, holes are injected in
the charge-storage insulating film 103, without being blocked by
the barrier, and the holes are not trapped in the tunnel insulating
film 102. As a result, the saturation of erase characteristics due
to the hole trap can be suppressed, and good erase characteristics
can be obtained.
In addition, in the present embodiment, as shown in FIG. 3C, at the
time of charge retention, the oxynitride film 102c and oxide film
102d function as a barrier against holes, and therefore the
degradation in charge retention characteristics can be
suppressed.
As a result, in the present embodiment, it is possible to obtain
the memory cell transistor having good charge erase characteristics
and charge retention characteristics.
As shown in FIG. 3B and FIG. 3C, in the case of (B) (corresponding
to (B) in FIG. 2), the barrier against holes from the low electric
field side becomes small at the time of the erase operation, and
thereby the erase speed is improved. In the case of (C)
(corresponding to (C) in FIG. 2), the energy barrier on the tunnel
insulating film 102 side against the holes accumulated in the
charge-storage insulating film 103 becomes higher, and thereby the
charge retention characteristics at the time of erase are
improved.
Next, referring to FIG. 1A, FIG. 1B, and FIG. 4A and FIG. 4B
through FIG. 7A and FIG. 7B, a basic manufacturing method of the
semiconductor device according to the first embodiment is
schematically described.
FIG. 4A through FIG. 7A are cross-sectional views taken along the
bit line direction of the semiconductor device according to the
first embodiment of the invention, and FIG. 4B through FIG. 7B are
cross-sectional views taken along the word line direction of the
semiconductor device according to the first embodiment of the
invention.
First, as shown in FIG. 4A and FIG. 4B, the surface of a
semiconductor substrate 101, in which desired impurities are doped,
is exposed to an oxygen atmosphere at 700.degree. C. Thereby, a
silicon oxide film with a thickness of about 1.5 nm is formed as an
oxide film (first region) 102a. In the meantime, the oxide film
102a may be formed by CVD. As regards the conditions for forming
the oxide film 102a by CVD, for example, dicyclosilane and nitrous
oxide gas is used as a material gas, and the temperature for film
formation is set at 600.degree. C. to 850.degree. C.
Subsequently, a silicon nitride film with a thickness of about 3.5
nm is deposited by ALD (Atomic Layer Deposition). Thereafter, at
least a surface region of the silicon nitride film is oxidized in
an atmosphere containing an oxygen radical at a substrate
temperature of 700.degree. C., and a silicon nitride film and a
silicon oxynitride film having a nitrogen concentration
distribution as shown in FIG. 2 are formed as a nitride film
(second region) 102b and an oxynitride film (fourth region) 102c.
In the meantime, the silicon nitride film may be formed by CVD. As
regards the conditions for forming the nitride film 102b by CVD,
for example, dichlorsilane and nitrous oxide gas is used as a
material gas, and the film formation is performed in a furnace at
600.degree. C. to 850.degree. C. Then, using dichlorsilane and
ozone gas, ALD is performed at a substrate temperature of
550.degree. C., and a silicon oxide film with a thickness of about
0.5 nm is deposited as an oxide film (third region) 102d. In the
meantime, the oxide film 102d may be formed by CVD. As regards the
conditions for forming the oxide film 102d by CVD, for example,
dichlorsilane and nitrous oxide gas is used as a material gas, and
the film formation is performed in a furnace at 600.degree. C. to
850.degree. C.
In this manner, the tunnel insulating film 102 having the
multilayer structure with a thickness of about 6 nm, which
comprises the oxide film 102a, nitride film 102b, oxynitride film
102c and oxide film 102d, is formed.
Subsequently, using CVD, a silicon nitride film with a thickness of
about 5 nm, which becomes a charge-storage insulating film 103, is
deposited. Further, a process mask material 108 is deposited on the
charge-storage insulating film 103 by CVD.
Next, as shown in FIG. 5A and FIG. 5B, by an RIE (Reactive Ion
Etching) method using a resist mask (not shown), the process mask
material 108, charge-storage insulating film 103 and tunnel
insulating film 102 are successively etched. Further, the exposed
semiconductor substrate is etched to a depth of about 100 nm, and
an element isolation trench 107a is formed.
Following the above, as shown in FIG. 6A and FIG. 6B, a silicon
oxide film is formed by a coating method, and the silicon oxide
film is planarized by CMP (Chemical Mechanical Polishing), thereby
forming an element isolation insulating film 107. Then, the process
mask material 108 is removed, and an alumina film with a thickness
of about 13 nm, which becomes a block insulating film 104, is
deposited by ALD. Subsequently, using CVD, a polycrystalline
silicon doped with impurities, which becomes a control gate
electrode 105, is deposited, and a process mask material 109 is
deposited on the control gate electrode 105.
Next, as shown in FIG. 7A and FIG. 7B, by an RIE method using a
resist mask (not shown), the process mask material 109, control
gate electrode film 105, block insulating film 104 and
charge-storage insulating film 103 are successively etched, and a
plurality of gate structures each having a width and an interval of
about 20 nm are formed. At this time, the surface of the tunnel
insulating film 102 is exposed. Then, using ion implantation and
thermal anneal, impurity diffusion layers (not shown) for a
source/drain are formed.
Next, as shown in FIG. 1A and FIG. 1B, the process mask material
109 is removed, and a silicon oxide film, which becomes an
interlayer insulating film 106, is formed by a coating method and
then planarized by CMP.
Thereafter, using well-known art, a wiring layer (not shown), etc.
are formed, and a nonvolatile semiconductor memory device is
completed.
According to the above-described first embodiment, by oxidizing the
silicon nitride film formed on the oxide film 102a, the oxynitride
film 102c having a desired nitrogen concentration distribution can
be formed between the nitride film 102b and the oxide film
102d.
The formation of the silicon oxynitride film 102c prevents direct
contact between the nitride film 102b and the oxide film 102d. This
relaxes a stress due to the differences in inter-lattice distance
and film expansion coefficient between the nitride film 102b and
the oxide film 102d, which are caused by direct contact between the
nitride film 102b and the oxide film 102d. In addition, defects
occurring at the interface between the nitride film 102b and oxide
film 102d can be reduced. Accordingly, at the time of the write
operation, the number of electrons, which are trapped in the tunnel
insulating film 102 when electrons are to be accumulated in the
charge-storage insulating film 103 from the semiconductor substrate
101 via the tunnel insulating film 102, is decreased. Besides, at
the time of the erase operation, the number of holes, which are
trapped in the tunnel insulating film 102 when holes are to be
injected in the charge-storage insulating film 103 from the
semiconductor substrate 101 via the tunnel insulating film 102, is
decreased. Therefore, it is possible to reduce the problem that
trapped electrons or holes leak to the semiconductor substrate 101,
and it is possible to improve the charge retention
characteristics.
As a result, in the present embodiment, it is possible to obtain
the memory cell transistor having good charge erase characteristics
and charge retention characteristics.
Next, the basic structures of semiconductor devices according to
modifications of the first embodiment are schematically described
with reference to FIG. 8, FIG. 9A, FIG. 9B, FIG. 10, FIG. 11A, FIG.
11B, FIG. 12, FIG. 13A and FIG. 13B. In each of these Figures, a
nitrogen concentration distribution (A) is a distribution in the
case where the nitrogen concentration decreases at a constant
ratio. A distribution (B) is a distribution in the case where the
quantity of nitrogen is greater and the quantity of oxygen is
smaller than in the case of the distribution (A). A distribution
(C) is a distribution in the case where the quantity of nitrogen is
smaller and the quantity of oxygen is greater than in the case of
the distribution (A).
(Modification 1)
FIG. 8 shows a cross-sectional view which schematically shows the
structure of a tunnel insulating film 102 of Modification 1 of the
first embodiment of the invention, and a nitrogen concentration
distribution in the depth direction of an oxynitride film 102f.
As shown in FIG. 8, in Modification 1, like the above-described
first embodiment, the tunnel insulating film 102 includes an oxide
film 102a (first region) and an oxide film 102d (third region). The
region, which corresponds to the nitride film 102b and oxynitride
film 102c in the first embodiment, is formed as the oxynitride film
(silicon oxynitride film) 102f. The other basic structure of
Modification 1 is the same as that of the first embodiment.
The nitrogen concentration in the oxynitride film 102f decreases
from the boundary between the oxide film 102a and the oxynitride
film 102f toward the boundary between the oxide film 102d and the
oxynitride film 102f. Conversely, the oxygen concentration
increases from the boundary between the oxide film 102a and the
oxynitride film 102f toward the boundary between the oxide film
102d and the oxynitride film 102f. The region, in which the
oxynitride film 102f is formed, virtually comprises two regions.
The region having a nitrogen concentration of a predetermined value
or more (or an oxygen concentration of a predetermined value or
less) corresponds to a second region 102f1. The region having a
nitrogen concentration lower than the predetermined value (or an
oxygen concentration higher than the predetermined value)
corresponds to a fourth region 102f2.
FIG. 9A is an energy band diagram at a time of a charge erase
operation of the memory cell transistor of Modification 1 of the
first embodiment of the invention, and FIG. 9B is an energy band
diagram at a time after the charge erase (at a time of charge
retention) of the memory cell transistor of Modification 1 of the
first embodiment of the invention.
As shown in FIG. 9A, at the time of the charge erase operation,
like the above-described embodiment, the barrier as shown in FIG.
3A is not present, and holes are injected in the charge-storage
insulating film 103, without being trapped in the potential well.
Thereby, like the first embodiment, the saturation of erase
characteristics due to the hole trap can be suppressed, and good
erase characteristics can be obtained.
In addition, as shown in FIG. 9B, at the time of charge retention,
the oxynitride film 102f and oxide film 102d function as a barrier
against holes, and therefore the degradation in charge retention
characteristics can be suppressed.
(Modification 2)
FIG. 10 shows a cross-sectional view of a tunnel insulating film
102 in Modification 2 of the first embodiment of the invention, and
a nitrogen concentration distribution in the depth direction of a
nitride film 102b and an oxynitride film 102g.
As shown in FIG. 10, in Modification 2, like the above-described
first embodiment, the tunnel insulating film 102 includes an oxide
film 102a (first region) and a nitride film 102b (second region).
The region, which corresponds to the oxynitride film 102c and oxide
film 102d in the above-described first embodiment, is formed as the
oxynitride film (silicon oxynitride film) 102g. The other basic
structure of Modification 2 is the same as that of the first
embodiment.
The nitrogen concentration in the oxynitride film 102g decreases
from the boundary between the nitride film 102b and oxynitride film
102g toward the boundary between the charge-storage insulating film
103 and the oxynitride film 102g. Conversely, the oxygen
concentration increases from the boundary between the nitride film
102b and oxynitride film 102g toward the boundary between the
charge-storage insulating film 103 and the oxynitride film 102g.
The region, in which the oxynitride film 102g is formed, virtually
comprises two regions. The region having an oxygen concentration of
a predetermined value or more (or a nitrogen concentration of a
predetermined value or less) corresponds to a third region 102g1.
The region having an oxygen concentration lower than the
predetermined value (or a nitrogen concentration higher than the
predetermined value) corresponds to a fourth region 102g2.
FIG. 11A is an energy band diagram at a time of a charge erase
operation of the memory cell transistor of Modification 2 of the
first embodiment of the invention, and FIG. 11B is an energy band
diagram at a time after the charge erase (at a time of charge
retention) of the memory cell transistor of Modification 2 of the
first embodiment of the invention.
As shown in FIG. 11A, at the time of the charge erase operation,
like the above-described embodiment, the barrier as shown in FIG.
3A is not present, and holes are injected in the charge-storage
insulating film 103, without being trapped in the potential well.
Thereby, like the first embodiment, the saturation of erase
characteristics due to the hole trap can be suppressed, and good
erase characteristics can be obtained.
In addition, as shown in FIG. 11B, at the time of charge retention,
the oxynitride film 102g functions as a barrier against holes, and
therefore the degradation in charge retention characteristics can
be suppressed.
(Modification 3)
FIG. 12 shows a cross-sectional view of a tunnel insulating film in
Modification 3 of the first embodiment of the invention, and a
nitrogen concentration distribution in the depth direction of an
oxynitride film 102h.
As shown in FIG. 12, in Modification 3, like the above-described
first embodiment, the tunnel insulating film 102 includes an oxide
film 102a. The region, which corresponds to the nitride film 102b,
oxynitride film 102c and oxide film 102d in the above-described
first embodiment, is formed as the oxynitride film (silicon
oxynitride film) 102h. The other basic structure of Modification 3
is the same as that of the first embodiment.
The nitrogen concentration in the oxynitride film 102h decreases
from the boundary between the oxide film 102a and the oxynitride
film 102h toward the boundary between the charge-storage insulating
film 103 and the oxynitride film 102h. Conversely, the oxygen
concentration increases from the boundary between the oxide film
102a and the oxynitride film 102h toward the boundary between the
charge-storage insulating film 103 and the oxynitride film 102h.
The region, in which the oxynitride film 102h is formed, virtually
comprises three regions. The region having a nitrogen concentration
of a predetermined value or more (or an oxygen concentration of a
predetermined value or less) corresponds to a second region 102h1.
The region having an oxygen concentration of a predetermined value
or more (or a nitrogen concentration of a predetermined value or
less) corresponds to a third region 102h2. The region between the
second region 102h1 and third region 102h2 corresponds to a fourth
region 102h3.
FIG. 13A is an energy band diagram at a time of a charge erase
operation of the memory cell transistor of Modification 3 of the
first embodiment of the invention, and FIG. 13B is an energy band
diagram at a time after the charge erase (a time of charge
retention) of the memory cell transistor of Modification 3 of the
first embodiment of the invention.
As shown in FIG. 13A, at the time of the charge erase operation,
like the above-described embodiment, the barrier as shown in FIG.
3A is not present, and holes are injected in the charge-storage
insulating film 103, without being trapped in the potential well.
Thereby, like the first embodiment, the saturation of erase
characteristics due to the hole trap can be suppressed, and good
erase characteristics can be obtained.
In addition, as shown in FIG. 13B, at the time of charge retention,
the oxynitride film 102h functions as a barrier against holes, and
therefore the degradation in charge retention characteristics can
be suppressed.
In each of the above-described Modifications, the silicon nitride
film, which is formed on the oxide film 102a, is oxidized by
controlling the amount of oxidation. Thereby, the silicon
oxynitride films having the nitrogen concentration distributions as
shown in FIG. 8, FIG. 10 and FIG. 12 can be formed.
In addition, the oxidation temperature at the time of oxidizing the
silicon nitride film formed on the oxide film 102a is controlled
from low temperatures to high temperatures (700.degree. C. or
above). Thereby, the silicon oxynitride films having the nitrogen
concentration distributions as indicated by (B), (A) or (C) in FIG.
2, FIG. 8, FIG. 10 and FIG. 12 can be formed.
In the above-described embodiment, by oxidizing the nitride film,
the oxynitride film having the desired nitrogen concentration
distribution is formed.
However, an oxynitride film having a desired nitrogen concentration
distribution can be formed by a manufacturing method using ALD. An
example of this method of formation is described below.
First, using Si source gas (e.g. SiH.sub.2Cl.sub.2), silicon for a
1-atomic layer is formed. Then, active oxygen (e.g. O.sub.2
radical, O radical, O.sub.3, etc.) is supplied at a flow rate x,
thereby oxidizing the silicon layer. Subsequently, a nitride gas
(e.g. NH radical, NH.sub.3, etc.) is supplied at a flow rate y,
thereby nitriding the silicon oxide film. Thus, a silicon
oxynitride film is formed. Then, a silicon layer for a 1-atomic
layer is formed on the oxynitride film, and by properly varying the
flow rate x and flow rate y, an oxynitride film with the varied
nitrogen concentration and oxygen concentration is formed. In this
manner, oxynitride films with varied concentrations are deposited
until a desired film thickness is obtained. Thereby, an oxynitride
film having a desired nitrogen concentration distribution and a
desired oxygen concentration distribution can be formed.
In the above embodiment, the nitride film 102b is formed by CVD.
Alternatively, the nitride film 102b may be formed by directly
subjecting the oxide film 102a, which is formed with a large
thickness, to thermal nitridation in an ammonia gas atmosphere. In
this case, since hydrogen is contained in the nitride film 102b,
the trap density of holes decreases. Accordingly, the erasure
saturation phenomenon can further be suppressed. Besides, the
nitride film 102b may be formed by directly subjecting the oxide
film 102a, which is formed with a large thickness, to nitridation
by a plasma using a nitrogen-based gas containing hydrogen atoms,
for instance, NH.sub.3 gas. In the meantime, the nitride film 102b
may be formed by nitriding the oxide film 102a by a plasma using a
mixture gas of a noble gas and a nitride-based gas. In the case of
using the plasma, nitridation at low temperatures can be performed,
and thus the diffusion of nitrogen into the oxide film 102a can be
suppressed and the increase in low-electric-field leak current can
be suppressed.
Second Embodiment
Next, a description is given of the structure of a memory cell
transistor of a flash memory according to a second embodiment of
the invention. FIG. 14A is a cross-sectional view taken in the bit
line direction of the memory cell transistor of the flash memory
according to the second embodiment, and FIG. 14B is a
cross-sectional view taken in the word line direction of the memory
cell transistor of the flash memory according to the second
embodiment.
As shown in FIG. 14A and FIG. 14B, a tunnel insulating film 102 is
formed on the surface of a semiconductor substrate (silicon
substrate) 101. A charge-storage insulating film 103 is formed on
the surface of the tunnel insulating film 102. A block insulating
film 104 is formed on the surface of the charge-storage insulating
film 103. An element isolation insulating film 107 is formed
between neighboring memory cell transistors MT. A channel region CH
is formed on a surface of the semiconductor substrate 101, which is
sandwiched between the element isolation insulating films 107. A
control gate electrode 105 is formed on the surface of the block
insulating film 104 and element isolation insulating film 107. A
gate voltage for data write, erase and read of the memory cell
transistor MT is applied to the control gate electrode 105. An
interlayer insulating film 106 is formed so as to cover the memory
cell transistor MT.
The tunnel insulating film 102 comprises an oxide film (first
region) 102a which is formed on the surface of the semiconductor
substrate 101; an oxynitride film (fifth region) 102e which is
formed on the surface of the oxide film 102a; a nitride film
(second region) 102b which is formed on the surface of the
oxynitride film 102e; an oxynitride film (fourth region) 102c which
is formed on the surface of the nitride film 102b; and an oxide
film (third region) 102d which is formed on the surface of the
oxynitride film 102c.
The oxynitride film 102e, as described above, is formed between the
oxide film 102a and nitride film 102b. Accordingly, the oxide film
102a and nitride film 102b are not in direct contact. In addition,
the oxynitride film 102c is formed between the nitride film 102b
and oxide film 102d. Thus, the nitride film 102b and oxide film
102d are not in direct contact.
Each of the oxide film 102a and oxide film 102d is, for instance, a
silicon oxide film containing silicon and oxygen as main
components. The nitride film 102b is, for instance, a silicon
nitride film containing silicon and nitrogen as main components.
Each of the oxynitride film 102c and oxynitride film 102e is, for
instance, a silicon oxynitride film containing silicon, nitrogen
and oxygen as main components.
In the tunnel insulating film 102, only one of the oxynitride film
102c and oxynitride film 102e may be formed. In this case, in order
to enhance the advantageous effect of the second embodiment, which
will be described later, it is desirable to form only the
oxynitride film 102e which is closer to the semiconductor substrate
101.
The thicknesses of the oxynitride film 102c and oxynitride film
102e may be equal or different. In the case of the latter, in order
to enhance the advantageous effect of the second embodiment, which
will be described later, it is desirable to make the thickness of
the oxynitride film 102e greater than the thickness of the
oxynitride film 102c.
FIG. 15 shows a cross-sectional view of the tunnel insulating film
102 in the second embodiment of the invention, and a nitrogen
concentration distribution in the depth direction of the oxynitride
film 102e, nitride film 102b and oxynitride film 102c. FIG. 16
shows a cross-sectional view of a tunnel insulating film 110 of a
comparative example of the second embodiment of the invention, and
a nitrogen concentration distribution in the depth direction of a
nitride film 102b.
As shown in FIG. 15, in the second embodiment, it is desirable that
the thickness of each of the oxynitride film 102e and oxynitride
film 102c be, e.g. about 1 nm. If the nitrogen concentration of the
nitride film 102b is assumed to be 1, the nitrogen concentration of
each of the oxynitride film 102e and oxynitride film 102c is, e.g.
0.6, relatively. The region in which the nitrogen concentration is
0.6 is present in a range of about 1 nm, which corresponds to the
thickness of each of the oxynitride film 102e and oxynitride film
102c. Specifically, the nitrogen concentration of the tunnel
insulating film 102 sharply increases from the boundary between the
oxynitride film 102e and oxide film 102a and from the boundary
between the oxynitride film 102c and oxide film 102d, and is kept
constant up to the boundary between the oxynitride film 102e and
nitride film 102b and the boundary between the oxynitride film 102c
and nitride film 102b. Then, the nitrogen concentration gently
increases from the boundary between the oxynitride film 102e and
nitride film 102b and the boundary between the oxynitride film 102c
and nitride film 102b, toward a central region in film thickness of
the nitride film 102b. In other words, the nitrogen concentration
in the tunnel insulating film 102 in the second embodiment varies
stepwise from the oxide film 102a and oxide film 102d toward the
nitride film 102b.
On the other hand, as shown in FIG. 16, in the case where no
oxynitride film is formed in the tunnel insulating film 102, if the
nitrogen concentration of the nitride film 102b is assumed to be 1,
there are regions with a nitrogen concentration of about 0.6
between the nitride film 102b and oxide film 102a and between the
nitride film 102b and oxide film 102d. However, in the comparative
example, the nitrogen concentration in the tunnel insulating film
102 gently increases from the boundary between the nitride film
102b and oxide film 102a and from the boundary between the nitride
film 102b and oxide film 102d, toward a central region in film
thickness of the nitride film 102b. Accordingly, in the comparative
example, unlike the second embodiment, the nitrogen concentration
does not vary stepwise.
The composition ratio between nitrogen and oxygen of each of the
oxynitride films 102e and 102c should desirably be
silicon:oxygen:nitrogen=3:6:4 in terms of an atomic number ratio,
in the case where silicon oxide in stoichiometric composition and
silicon nitride in stoichiometric composition are present in a
fifty-fifty ratio. In the meantime, each of the oxynitride films
102e and 102c may be oxygen-rich or nitrogen-rich, relative to this
atomic number ratio. In addition, the silicon composition ratio of
each of the oxynitride films 102e and 102c may be silicon-rich or
silicon-poor.
The materials of the tunnel insulating film 102 may variously be
altered. For example, each of the oxide films 102a and 102d,
nitride film 102b and oxynitride films 102e and 102c may be formed
of a material including germanium. Concrete examples of the
combination of materials may be, in place of the combination of
silicon oxide film 102a/silicon oxynitride film 102e/silicon
nitride film 102b/silicon oxynitride film 102c/silicon oxide film
102d, a combination of silicon-germanium oxide
film/silicon-germanium oxynitride film/silicon-germanium nitride
film/silicon-germanium oxynitride film/silicon-germanium oxide
film, and a combination of germanium oxide film/germanium
oxynitride film/germanium nitride film/germanium oxynitride
film/germanium oxide film.
Next, referring to FIG. 14A and FIG. 14B, a manufacturing method of
the memory cell transistor according to the present embodiment is
described.
First, an oxide film (first silicon oxide film) 102a is formed on a
semiconductor substrate 101 by CVD. As regards the conditions for
forming the oxide film 102a by CVD, for example, dichlorsilane and
nitrous oxide gas is used as a material gas, and the temperature
for film formation is set at 600.degree. C. to 850.degree. C. In
the meantime, the oxide film 102a may be formed of a
thermally-oxidized film by an oxidizing atmosphere gas.
Subsequently, a silicon oxynitride film, which becomes an
oxynitride film 102e, is formed on the oxide film 102a by CVD. As
regards the conditions for forming the oxynitride film 102e by CVD,
for example, dichlorsilane, nitrous oxide and ammonia are used as
material gases, and these material gases are introduced at the same
time into a reaction chamber at 600.degree. C. to 850.degree. C. By
varying the ratio in flow rate between the dichlorsilane and
ammonia, the atomic number ratio between oxygen and nitrogen in the
oxynitride film 102e can be controlled.
Next, a silicon nitride film, which becomes a nitride film 102b, is
formed on the oxynitride film 102e by CVD. As regards the
conditions for forming the nitride film 102b by CVD, for example,
dichlorsilane and nitrous oxide gas is used as a material gas, and
the film formation is performed in a furnace at 600.degree. C. to
850.degree. C.
Subsequently, a silicon oxynitride film, which becomes an
oxynitride film 102c, is formed on the nitride film 102b by CVD.
The conditions for forming the oxynitride film 102c by CVD and the
method of controlling the atomic number ratio are the same as in
the case of the oxynitride film 102e.
Next, an oxide film 102d is formed on the oxynitride film 102c by
CVD. As regards the conditions for forming the oxide film 102d by
CVD, for example, dichlorsilane and nitrous oxide gas is used as a
material gas, and the temperature for film formation is set at
600.degree. C. to 850.degree. C.
Next, a charge-storage layer 103 is formed on the oxide film 102d.
As regards the conditions for forming the charge-storage layer 103,
for example, trimethyl aluminum and water vapor are used as
material gas, and the film formation is performed in a furnace at
about 600.degree. C. Under these conditions, the charge-storage
layer 103, which is composed of an aluminum oxide film, is
formed.
Thereafter, using a generally known method, a block insulating film
104, an element isolation insulating film 107, a control gate
electrode 105 and an interlayer insulating film 106 are formed.
As regards the above-described oxynitride film 102e and oxynitride
film 102c, the materials and the method of formation are not
limited and may be variously altered. In the above description, the
oxynitride film 102e and oxynitride film 102c are formed by CVD
with use of dichlorsilane, ammonia and nitrous oxide. As the
silicon material gas, the dichlorsilane may be replaced with
monosilane or disilane. In addition, as the oxygen material gas,
the nitrous oxide may be replaced with, for instance, oxygen,
ozone, or nitrogen monoxide. Besides, as the formation method, the
CVD method may be replaced with, for instance, an ALD method in
which 1-atom layers are deposited one by one.
The oxynitride film 102e may also be formed by nitriding the oxide
film 102a. Specifically, after the oxide film 102a is formed,
ammonia, nitrogen monoxide or nitrous oxide is subjected to heat
treatment at about 500.degree. C. to 1100.degree. C. By this heat
treatment, the surface of the oxide film 102a is nitrided, and the
oxynitride film 102e can be formed. Alternatively, after the oxide
film 102a is formed, nitrogen or ammonia is excited by microwaves
or the like, and the generated nitrogen or ammonia radical is
introduced into a reaction chamber. By this process, the surface of
the oxide film 102a is nitrided, and the oxynitride film 102e can
be formed.
On the other hand, the oxynitride film 102c may also be formed by
oxidizing the nitride film 102b. Specifically, after the nitride
film 102b is formed, a gas containing an oxidizing gas such as
oxygen or water vapor is introduced into a reaction chamber and
subjected to heat treatment at about 600.degree. C. to 1100.degree.
C. By this heat treatment, the surface of the nitride film 102b is
oxidized, and the oxynitride film 102c can be formed.
Alternatively, after the nitride film 102b is formed, an oxidizing
gas, such as oxygen or nitrogen monoxide, is excited by microwaves
or the like, and the generated oxidizing radical is introduced into
a reaction chamber. By this process, the surface of the nitride
film 102b is nitrided, and the oxynitride film 102c can be
formed.
According to the second embodiment, the oxynitride film 102e is
formed between the nitride film 102b and oxide film 102a, and the
oxynitride film 102c is formed between the nitride film 102b and
oxide film 102d.
The presence of the oxynitride film 102e and oxynitride film 102c
prevents direct contact between the nitride film 102b and the oxide
film 102a and direct contact between the nitride film 102b and the
oxide film 102d. Thereby, it is possible to relax a stress due to
the differences in inter-lattice distance and film expansion
coefficient between the silicon nitride film 102b and silicon oxide
film 102a and between the silicon nitride film 102b and silicon
oxide film 102d, which are caused by direct contact between the
silicon nitride film 102b and the silicon oxide film 102a and
direct contact between the silicon nitride film 102b and the
silicon oxide film 102d. In addition, it is possible to reduce
defects occurring at the interface between the silicon nitride film
102b and silicon oxide film 102a and the interface between the
silicon nitride film 102b and silicon oxide film 102d. Accordingly,
at the time of the write operation, the number of electrons, which
are trapped in the tunnel insulating film 102 when electrons are to
be accumulated in the charge-storage layer 103 from the
semiconductor substrate 101 via the tunnel insulating film 102, is
decreased. Besides, at the time of the erase operation, the number
of holes, which are trapped in the tunnel insulating film 102 when
holes are to be injected in the charge-storage layer 103 from the
semiconductor substrate 101 via the tunnel insulating film 102, is
decreased. Therefore, it is possible to reduce the problem that
trapped electrons or holes leak to the semiconductor substrate 101,
and it is possible to improve the charge retention characteristics
of the memory cell transistor MT.
In the tunnel insulating film 102, the silicon nitride film 102b is
positioned between the silicon oxide film 102a and the silicon
oxide film 102d. In general, a silicon nitride film has such
characteristics that the silicon nitride film has a less barrier
height against holes than a silicon oxide film. Thus, the formation
of the silicon nitride film 102b between the silicon oxide film
102a and silicon oxide film 102d can improve the efficiency of
injection of holes from the semiconductor substrate 101 into the
charge-storage layer 103 via the tunnel insulating film 102.
Third Embodiment
A third embodiment of the invention relates to a nonvolatile
semiconductor memory device having a 3-D structure, which is formed
by using a 3-D multilayer technology BiCS (Bit Cost Scalable).
Referring to FIG. 17A, FIG. 17B, FIG. 18A and FIG. 18B, the basic
structure of a semiconductor device according to the third
embodiment is schematically described.
FIG. 17A is a cross-sectional view taken in the channel length
direction of a memory cell transistor according to the third
embodiment of the invention, and FIG. 17B is a perspective view of
the memory cell transistor according to the third embodiment.
As shown in FIG. 17A and FIG. 17B, a tunnel insulating film 202 is
formed on the surface, i.e. the peripheral surface, of a columnar
semiconductor region (silicon region) 201. A charge-storage
insulating film 203 is formed on the surface of the tunnel
insulating film 202. A block insulating film 204 is formed on the
surface of the charge-storage insulating film 203. A control gate
electrode 205 is formed on a surface of the block insulating film
204. The block insulating film 204 and control gate electrode 205
are covered with an interlayer insulating film 206.
The tunnel insulating film 202 comprises an oxide film (first
region) 202a which is formed on the surface of the semiconductor
region 201; a nitride film (second region) 202b which is formed on
the surface of the oxide film 202a; an oxynitride film (fourth
region) 202c which is formed on the surface of the nitride film
202b; and an oxide film (third region) 202d which is formed on the
surface of the oxynitride film 202c.
Each of the oxide film 202a and oxide film 202d is, for instance, a
silicon oxide film containing silicon and oxygen as main
components. The nitride film 202b is, for instance, a silicon
nitride film containing silicon and nitrogen as main components.
The oxynitride film 202c is, for instance, a silicon oxynitride
film containing silicon, nitrogen and oxygen as main
components.
The width of each memory cell transistor is about 50 nm, and also
the interval of neighboring memory cell transistors is about 50
nm.
FIG. 18A is a cross-sectional view, taken in the channel length
direction, showing the structure in which the memory cell
transistor according to the third embodiment of the invention is
provided continuous in the vertical direction (channel length
direction), and FIG. 18B is a cross-sectional view, taken in a
direction perpendicular to the channel length direction, showing
the structure in which the memory cell transistor according to the
third embodiment of the invention is provided continuous in the
vertical direction (channel length direction).
As shown in FIG. 18A and FIG. 18B, the memory cell transistors,
which have been described with reference to FIG. 17A and FIG. 17B,
are successively stacked on the semiconductor substrate 200.
FIG. 18A and FIG. 18B show the case in which two layers of memory
cell transistors are formed. However, any number of layers of
memory cell transistors may be formed, where necessary.
In the tunnel insulating film 202 of the third embodiment, like the
above-described first embodiment, the oxynitride film 202c is
formed between the nitride film 202b and oxide film 202d. Thus,
like the first embodiment, holes are injected in the charge-storage
insulating film 203, without being blocked by the barrier, and the
holes are not trapped in the tunnel insulating film 202. Therefore,
the saturation of erase characteristics due to the hole trap can be
suppressed, and good erase characteristics can be obtained.
In addition, in the third embodiment, like the first embodiment, at
the time of charge retention, the oxynitride film 202c and oxide
film 202d function as a barrier against holes, and therefore the
degradation in charge retention characteristics can be
suppressed.
As a result, in the third embodiment, like the first embodiment, it
is possible to obtain the memory cell transistor having good charge
erase characteristics and charge retention characteristics.
Next, referring to FIG. 18A and FIG. 18B through FIG. 21A and FIG.
21B, a basic manufacturing method of the semiconductor device
according to the third embodiment is schematically described.
FIG. 19A through FIG. 21A are cross-sectional views taken along the
channel length direction of the semiconductor device according to
the third embodiment of the invention, and FIG. 19B through FIG.
21B are cross-sectional views taken along a direction perpendicular
to the channel length direction of the semiconductor device
according to the third embodiment of the invention.
First, as shown in FIG. 19A and FIG. 19B, a silicon oxide film with
a thickness of about 50 nm, which becomes an interlayer insulating
film 206, and an impurity-doped silicon film with a thickness of
about 50 nm, which becomes a control gate electrode 205, are
alternately deposited by CVD by a desired number of times. As the
material of the control gate electrode 205, for example, use may be
made of a metallic material such as tantalum nitride.
Next, as shown in FIG. 20A and FIG. 20B, by an RIE method using a
resist mask (not shown), the interlayer insulating film 206 and
control gate electrode 205 are selectively etched away, and the
semiconductor substrate 200 is exposed. Thereby, a cylindrical
trench 207 with a diameter of about 60 nm is formed in the
multilayer structure of the interlayer insulating films 206 and
control gate electrodes 205. Thereafter, for example, an alumina
film with a thickness of about 10 nm, which contains aluminum and
oxygen as main components and becomes a block insulating film 204,
is deposited by CVD on the inner wall of the trench 207.
Alternatively, the block insulating film 204 may be, for instance,
a silicon oxide film containing silicon and oxygen as main
components.
Then, as shown in FIG. 21A and FIG. 21B, a silicon nitride film
with a thickness of about 5 nm, which becomes a charge-storage
insulating film 203, is deposited by ALD. Subsequently, a silicon
oxide film with a thickness of about 4 nm is formed, and the
surface of the silicon oxide film is nitrided by about 1.5 nm by a
plasma using NH.sub.3 gas. Thus, a silicon oxynitride film having a
distribution in nitrogen concentration is formed. Thereby, the
oxide film 202d and oxynitride film 202c are formed. Thereafter, a
silicon oxynitride film with a thickness of about 2 nm, which
becomes a nitride film 202b, is formed by ALD, and a silicon oxide
film with a thickness of about 1.5 nm, which becomes an oxide film
202a, is formed by ALD. Thereby, a tunnel insulating film 202
having the multilayer structure, which comprises the oxide film
202a, nitride film 202b, oxynitride film 202c and oxide film 202d,
is formed.
Next, as shown in FIG. 18A and FIG. 18B, the block insulating film
204, charge-storage insulating film 203 and tunnel insulating film
202, which are formed on the bottom portion of the trench 207, and
the surface of the semiconductor substrate 200 are selectively
etched away by RIE using a resist mask (not shown). Then, an
impurity-doped silicon film, which becomes a channel region, is
deposited by CVD and is subjected to heat treatment in a nitrogen
atmosphere at about 600.degree. C., thereby forming a semiconductor
region 201. Thereafter, using well-known art, a wiring layer (not
shown), etc. are formed, and a nonvolatile semiconductor memory
device is completed.
According to the above-described third embodiment, by nitriding the
silicon nitride film formed on the surface of the block insulating
film 204, the oxynitride film 202c having a desired nitrogen
concentration distribution can be formed.
As a result, in the third embodiment, like the first embodiment, it
is possible to obtain the memory cell transistor having good charge
erase characteristics and charge retention characteristics.
In the above-described third embodiment, the tunnel insulating film
202 has the multilayer structure which comprises the oxide film
202a, nitride film 202b, oxynitride film 202c and oxide film 202d.
However, modifications, which are similar to the modifications of
the first embodiment, are also applicable to the third
embodiment.
In the above-described third embodiment, by nitriding the oxide
film, the oxynitride film having the desired nitrogen concentration
distribution is formed. However, an oxynitride film having a
desired nitrogen concentration distribution can be formed by a
manufacturing method using ALD. An example of this method of
formation is described below.
First, using Si source gas (e.g. SiH.sub.2Cl.sub.2), silicon for a
1-atomic layer is formed. Then, active oxygen (e.g. O.sub.2
radical, O radical, O.sub.3, etc.) is supplied at a flow rate x,
thereby oxidizing the silicon layer. Subsequently, a nitriding gas
(e.g. NH radical, NH.sub.3, etc.) is supplied at a flow rate y,
thereby nitriding the silicon oxide film. Thus, a silicon
oxynitride film is formed. Then, a silicon layer for a 1-atomic
layer is formed on the oxynitride film, and by properly varying the
flow rate x and flow rate y, an oxynitride film with the varied
nitrogen concentration and oxygen concentration is formed. In this
manner, oxynitride films with varied concentrations are deposited
until a desired film thickness is obtained. Thereby, an oxynitride
film having a desired nitrogen concentration distribution and a
desired oxygen concentration distribution can be formed.
In the third embodiment, the oxynitride film 202c is formed by
nitriding, with use of a plasma, the oxide film which is formed on
the surface of the block insulating film 204. Alternatively, the
oxynitride film 202c may be formed at the same time when the
nitride film 202b is formed by ALD. In an example of the formation
method, prior to the formation of the nitride film 202b, plasma
nitridation is performed using a nitriding gas such as NH.sub.3
gas, or thermal oxidation is performed at 700.degree. C. In this
case, since the oxynitride film 202c and nitride film 202b can be
formed successively without contact with an outside atmosphere, no
silicon oxide film is formed between the oxynitride film 202c and
nitride film 202b, and the energy barrier against holes is
prevented from increasing. Therefore, the erasure saturation
phenomenon can further be suppressed.
In an actual nonvolatile semiconductor memory device, a plurality
of memory cell transistors are arranged in the word line direction
and bit line direction. A typical example of the above-described
nonvolatile semiconductor memory device is a NAND type nonvolatile
memory which is configured such that a plurality of
series-connected memory cell transistors are provided between
select transistors.
Fourth Embodiment
A fourth embodiment of the invention relates to a nonvolatile
semiconductor memory device having a 3-D structure, which is formed
by using a 3-D multilayer technology BiCS.
In the above-described third embodiment, like the first embodiment,
a description has been given of the tunnel insulating film with the
4-layer structure comprising the silicon oxide film/silicon nitride
film/silicon oxynitride film/silicon oxide film. In the fourth
embodiment, like the second embodiment, the tunnel insulating film
has a 5-layer structure comprising a silicon oxide film/silicon
oxynitride film/silicon nitride film/silicon oxynitride
film/silicon oxide film.
Referring to FIG. 22A, FIG. 22B, FIG. 23A and FIG. 23B, the basic
structure of a semiconductor device according to the fourth
embodiment is schematically described.
FIG. 22A is a cross-sectional view, taken along the channel length
direction, showing the structure of the memory cell transistor
according to the fourth embodiment of the invention, and FIG. 22B
is a perspective view showing the structure of the memory cell
transistor according to the fourth embodiment of the invention.
As shown in FIG. 22A and FIG. 22B, a tunnel insulating film 202 is
formed on the surface, i.e. the peripheral surface, of a columnar
semiconductor region (silicon region) 201. A charge-storage
insulating film 203 is formed on the surface of the tunnel
insulating film 202. A block insulating film 204 is formed on the
surface of the charge-storage insulating film 203. A control gate
electrode 205 is formed on a surface of the block insulating film
204. The block insulating film 204 and control gate electrode 205
are covered with an interlayer insulating film 206.
The tunnel insulating film 202 comprises an oxide film (first
region) 202a which is formed on the surface of the semiconductor
region 201; an oxynitride film (fifth region) 202e which is formed
on the surface of the oxide film 202a; a nitride film (second
region) 202b which is formed on the surface of the oxynitride film
202e; an oxynitride film (fourth region) 202c which is formed on
the surface of the nitride film 202b; and an oxide film (third
region) 202d which is formed on the surface of the oxynitride film
202c.
Each of the oxide film 202a and oxide film 202d is, for instance, a
silicon oxide film containing silicon and oxygen as main
components. The nitride film 202b is, for instance, a silicon
nitride film containing silicon and nitrogen as main components.
Each of the oxynitride film 202e and oxynitride film 202c is, for
instance, a silicon oxynitride film containing silicon, nitrogen
and oxygen as main components.
The nitrogen concentration in the oxynitride film 202e and
oxynitride film 202c is the same as those in the oxynitride film
202e and oxynitride film 202c in the second embodiment.
The width of each memory cell transistor is about 50 nm, and also
the interval of neighboring memory cell transistors is about 50
nm.
FIG. 23A is a cross-sectional view, taken in the channel length
direction, showing the structure in which the memory cell
transistor according to the fourth embodiment of the invention is
provided continuous in the vertical direction (channel length
direction), and FIG. 23B is a cross-sectional view, taken in a
direction perpendicular to the channel length direction, showing
the structure in which the memory cell transistor according to the
fourth embodiment of the invention is provided continuous in the
vertical direction (channel length direction).
As shown in FIG. 23A and FIG. 23B, the memory cell transistors,
which have been described with reference to FIG. 22A and FIG. 22B,
are successively stacked on the semiconductor substrate 200.
FIG. 23A and FIG. 23B show the case in which two layers of memory
cell transistors are formed. However, any number of layers of
memory cell transistors may be formed, where necessary.
Next, referring to FIG. 23A and FIG. 23B through FIG. 26A and FIG.
26B, a basic manufacturing method of the semiconductor device
according to the fourth embodiment is schematically described.
FIG. 24A through FIG. 26A are cross-sectional views taken along the
channel length direction of the semiconductor device according to
the fourth embodiment of the invention, and FIG. 24B through FIG.
26B are cross-sectional views taken in a direction perpendicular to
the channel length direction of the semiconductor device according
to the fourth embodiment of the invention.
First, as shown in FIG. 24A and FIG. 24B, a silicon oxide film with
a thickness of about 50 nm, which becomes an interlayer insulating
film 206, and an impurity-doped silicon film with a thickness of
about 50 nm, which becomes a control gate electrode 205, are
alternately deposited by CVD on the surface of the semiconductor
substrate 200 by a desired number of times. As the material of the
control gate electrode 205, for example, use may be made of a
metallic material such as tantalum nitride.
Next, as shown in FIG. 25A and FIG. 25B, by an RIE method using a
resist mask (not shown), the interlayer insulating film 206 and
control gate electrode 205 are selectively etched away, and the
semiconductor substrate 200 is exposed. Thereby, a cylindrical
trench 207 with a diameter of about 60 nm is formed in the
multilayer structure of the interlayer insulating films 206 and
control gate electrodes 205. Thereafter, for example, an alumina
film with a thickness of about 10 nm, which contains aluminum and
oxygen as main components and becomes a block insulating film 204,
is deposited by CVD on the inner wall of the trench 207.
Alternatively, the block insulating film 204 may be, for instance,
a silicon oxide film containing silicon and oxygen as main
components.
Then, as shown in FIG. 26A and FIG. 26B, an alumina film, which
becomes a charge-storage insulating film 203, is deposited.
Subsequently, a silicon oxide film, which becomes an oxide film
202d, is formed, and a silicon oxynitride film, which becomes an
oxynitride film 202c, is formed by CVD on the surface of the oxide
film 202d. Thereafter, a silicon nitride film, which becomes a
nitride film 202b, is formed by CVD, and a silicon oxynitride film,
which becomes an oxynitride film 202e, is formed by CVD on the
surface of the nitride film 202b. Then, a silicon oxide film, which
becomes an oxide film 202a, is formed on the surface of the
oxynitride film 202e. Thereby, a tunnel insulating film 202 having
the multilayer structure, which comprises the oxide film 202a,
oxynitride film 202e, nitride film 202b, oxynitride film 202c and
oxide film 202d, is formed.
Next, as shown in FIG. 23A and FIG. 23B, the block insulating film
204, charge-storage insulating film 203 and tunnel insulating film
202, which are formed on the bottom portion of the trench 207, and
the surface of the semiconductor substrate 200 are selectively
etched away by RIE using a resist mask (not shown). Then, an
impurity-doped silicon film, which becomes a channel region, is
deposited by CVD and is subjected to heat treatment in a nitrogen
atmosphere at about 600.degree. C., thereby forming a semiconductor
region 201. Thereafter, using well-known art, a wiring layer (not
shown), etc. are formed, and a nonvolatile semiconductor memory
device is completed.
According to the fourth embodiment, like the above-described second
embodiment, the oxynitride film 202e is formed between the nitride
film 202b and oxide film 202a, and the oxynitride film 202c is
formed between the nitride film 202b and the oxide film 202d.
Therefore, like the second embodiment, it is possible to reduce the
problem that trapped electrons or holes leak to the semiconductor
substrate 101, and it is possible to improve the charge retention
characteristics.
In the tunnel insulating film 202, the silicon nitride film 202b is
formed between the silicon oxide film 202a and the silicon oxide
film 202d. Therefore, it is possible to improve the efficiency of
injection of holes from the semiconductor substrate 201 into the
charge-storage layer 203 via the tunnel insulating film 202.
In the above-described embodiment, the oxynitride film is formed by
CVD. However, the oxynitride film 202e can be formed by oxidizing
the surface of the nitride film 202b. Specifically, after the
nitride film 202b is formed, a gas containing an oxidizing gas such
as oxygen or water vapor is introduced into a reaction chamber and
subjected to heat treatment at about 600.degree. C. to 1100.degree.
C. By this heat treatment, the surface of the nitride film 202b is
oxidized, and the oxynitride film 202c can be formed.
Alternatively, after the nitride film 202b is formed, an oxidizing
gas, such as oxygen or nitrogen monoxide, is excited by microwaves
or the like, and the generated oxidizing radical is introduced into
a reaction chamber. By this process, the surface of the nitride
film 202b is oxidized, and the oxynitride film 202c can be formed.
In addition, like the above-described third embodiment, the
oxynitride film 202c may be formed by oxidizing the surface of the
nitride film 202b. Besides, like the third embodiment, the
oxynitride films 202c and 202e can be formed by a manufacturing
method using ALD.
Fifth Embodiment
A fifth embodiment of the invention relates to an example in which
the charge retention characteristics of a memory cell transistor MT
are improved by controlling a bias voltage at the time of a write
operation and at the time of an erase operation. A description of
the parts common to those in the second embodiment is omitted, and
only differences are described in detail.
First, the structure of a flash memory according to the present
embodiment is described. FIG. 27 is a block diagram of the flash
memory according to the embodiment.
As shown in FIG. 27, a flash memory 30 comprises a control circuit
31, a row decoder 32, a column decoder 33, a memory cell array 35,
and a sense amplifier S/A.
The control circuit 31 is configured to control the voltage values
of a gate voltage at the time of write, erase and read, and to
control addresses which are selected by the row decoder 32 and
column decoder 33.
The row decoder 32 is configured to select word lines WL0 to WL31
in accordance with the control of the control circuit 31.
The column decoder 33 is configured to select bit lines BL0 to BLm
in accordance with the control of the control circuit 31.
The memory cell array 35 comprises a plurality of blocks (Block
n-1, Block n, Block n+1, . . . ). The block (Block n) comprises a
plurality of memory cell transistors MT which are arrayed in a
matrix at intersections between the word lines WL0 to WL31 and the
bit lines BL0 to BLm.
The sense amplifier S/A is configured to amplify the data of the
memory cell transistors MT in each page, which is read out of the
bit lines BL0 to BLm.
Next, the structure of the memory cell transistor of the flash
memory according to the fifth embodiment is described. FIG. 28A is
a cross-sectional view, taken along the word line (WL) direction,
showing the memory cell transistor in the flash memory according to
the present embodiment, and FIG. 28B is a cross-sectional view,
taken along the bit line (BL) direction, showing the memory cell
transistor in the flash memory according to the present
embodiment.
As shown in FIG. 28A and FIG. 28B, the fifth embodiment differs
from the second embodiment in that the tunnel insulating film 102
has a 3-layer structure. Specifically, the tunnel insulating film
102 comprises an oxide film (first silicon oxide film) 102a, a
nitride film (silicon nitride film) 102b and an oxide film (second
silicon oxide film) 102d. The oxide film 102a is formed on the
semiconductor substrate 101. The nitride film 102b is formed on the
oxide film 102a. The oxide film 102d is formed on the nitride film
102b.
In the fifth embodiment, like the second embodiment, a silicon
oxynitride film may be formed at least between the oxide film 102a
and nitride film 102b or between the nitride film 102b and oxide
film 102d.
Next, the write operation of the flash memory according to the
fifth embodiment is described. FIG. 29 shows the variation with
time of the gate voltage at a time of data write in the flash
memory according to the fifth embodiment. FIG. 30A and FIG. 30B are
cross-sectional views illustrating operations at the time of data
write in the memory cell transistor of the flash memory according
to the fifth embodiment. At the time of the write operation, the
gate voltage, which is applied to the control gate electrode 105,
is controlled by the control circuit 31 shown in FIG. 27.
As shown in FIG. 29, in the case where the electrical effective
film thickness (T.sub.eff) of the tunnel insulating film 102 is,
e.g. 15 nm, the control circuit 31 applies, at the time of write, a
positive bias voltage of, e.g. 20 V to the control gate electrode
105 for one second, and thereafter applies a negative bias voltage
of, e.g. -4 V to the control gate electrode 105 for one second.
Specifically, in the write operation of the fifth embodiment, after
a first voltage (+V1) that is a positive bias is applied, a second
voltage (-V2:|V2|<|V1|) that is a negative bias and has a
smaller absolute value than the first voltage is applied.
When a positive bias voltage has been applied, as shown in FIG.
30A, electrons are accumulated in the charge-storage layer 103 from
the semiconductor substrate 101 via the tunnel insulating film 102,
and part of electrons are trapped in defects in the tunnel
insulating film 102. However, thereafter, by the application of a
negative bias voltage, as shown in FIG. 30B, the electrons trapped
in the tunnel insulating film 102 are extracted to the
semiconductor substrate 101 side. At this time, since the absolute
value of the negative bias voltage is less than that of the
previously applied positive bias voltage, the electrons in the
charge-storage layer 103 are not extracted, and only the electrons
trapped in the tunnel insulating film 102 are extracted.
Next, the erase operation of the flash memory according to the
fifth embodiment is described. FIG. 31 shows the variation with
time of the gate voltage at a time of data erase in the flash
memory according to the fifth embodiment. FIG. 32A and FIG. 32B are
cross-sectional views illustrating operations at the time of data
erase in the memory cell transistor of the flash memory according
to the fifth embodiment. At the time of the erase operation, the
gate voltage, which is applied to the control gate electrode 105,
is controlled by the control circuit 31 shown in FIG. 27.
As shown in FIG. 31, in the case where the electrical effective
film thickness (T.sub.eff) of the tunnel insulating film 102 is,
e.g. 15 nm, the control circuit 31 applies, at the time of erase, a
negative bias voltage of, e.g. -20 V to the control gate electrode
105 for one second, and thereafter applies a positive bias voltage
of, e.g. 4 V to the control gate electrode 105 for one second.
Specifically, in the erase operation of the fifth embodiment, after
a first voltage (-V3) that is a negative bias is applied, a second
voltage (+V4:|V4|<|V3|) that is a positive bias and has a
smaller absolute value than the first voltage is applied.
When a negative bias voltage has been applied, as shown in FIG.
32A, holes are introduced in the charge-storage layer 103 from the
semiconductor substrate 101 via the tunnel insulating film 102, and
part of holes are trapped in defects in the tunnel insulating film
102. However, thereafter, by the application of a positive bias
voltage, as shown in FIG. 32B, electrons are introduced in the
tunnel insulating film 102, and the introduced electrons and the
trapped holes cancel each other. At this time, since the absolute
value of the positive bias voltage is less than that of the
previously applied negative bias voltage, the electrons are not
injected in the charge-storage layer 103, and the electrons are
injected only into the tunnel insulating film 102 and the injected
electrons and the trapped holes cancel each other.
According to the fifth embodiment, the write operation and erase
operation are controlled by the control circuit 31.
Specifically, in the case of the write operation, after the
positive bias voltage is applied, the negative bias voltage, which
has a smaller absolute value than the positive bias voltage, is
applied. By executing this series of operations as the write
operation, the number of electrons, which are trapped in the tunnel
insulating film 102, can be reduced. Thus, it is possible to
prevent the trapped electrons from leaking to the semiconductor
substrate 101 after the write operation. Thereby, since the problem
of degradation in threshold can be solved, the charge retention
characteristics can be improved. Moreover, by executing this write
operation, even if defects occur not only at the interface but also
anywhere in the tunnel insulating film 102, and electrons are
trapped, the trapped electrons can be extracted and the charge
retention characteristics can be improved.
On the other hand, in the case of the erase operation, after the
negative bias voltage is applied, the positive bias voltage, which
has a smaller absolute value than the negative bias voltage, is
applied. By executing this series of operations as the erase
operation, the number of holes, which are trapped in the tunnel
insulating film 102, can be reduced. Thus, it is possible to
prevent the trapped holes from leaking to the semiconductor
substrate 101 after the erase operation. Thereby, since the problem
of degradation in threshold can be solved, the charge retention
characteristics can be improved. Furthermore, by executing this
erase operation, even if defects occur not only at the interface
but also anywhere in the tunnel insulating film 102 and holes are
trapped, electrons are injected to cancel the trapped holes, and
thereby the charge retention characteristics can be improved.
In the write operation and erase operation in the fifth embodiment,
the application time of the positive bias and the application time
of the negative bias are fixed. However, these application times
may be varied. For example, in the case of the write operation,
after the positive bias voltage is applied, the negative bias
voltage may be applied for a shorter time than the positive bias
voltage. Besides, in the case of the erase operation, after the
negative bias voltage is applied, the positive bias voltage may be
applied for a shorter time than the negative bias voltage.
Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects
is not limited to the specific details and representative
embodiments shown and described herein. Accordingly, various
modifications may be made without departing from the spirit or
scope of the general inventive concept as defined by the appended
claims and their equivalents.
* * * * *